POLLACK (POLLACHIUS-POLLACHIUS) STOCK SIZE DEVELOPMENT AND POTENTIAL INFLUENCE ON COD (GADUS-MORHUA) MARICULTURE IN A WEST NORWEGIAN FJORD

This study examines density-dependent mortality of pollack, a major co d predator, and cod, before and after juvenile cod were released on a large scale in Masfjorden, western Norway. The development in stock si ze, mortality by age group and diet are emphasised, and results for po llack and cod are compared. Population parameters for pollack are esti mated using a combination of the catch per unit effort (CPUE) method a nd a general cohort analysis technique. The estimates for cod originat e from a combination of the CPUE method and tag-release-recapture expe riments and have been published earlier. The diet studies are based on pollack and cod subdivided into predator size groups for four seasons at a period before large-scale juvenile cod releases. During this per iod, but also after releases, the mortality patterns, stomach analysis and the progression in prey (1-group) and predator (2+-group) abundan ce demonstrate cannibalism as well as that pollack and cod were feedin g on each other's juveniles in Masfjorden. Density-dependent predation mortality and growth seem to be an important regulatory mechanism for pollack and cod stocks in Masfjorden. This may explain the unsuccessf ul cod mariculture in this fjord.